With an increase in the recording density of magnetic disks, the distance between the magnetic disk serving as a recording medium and the head for use in recording of information or playback has become almost nil close to contact therebetween. The magnetic disk is provided over the surface thereof with a carbon protective film or lubricant film for the purpose of diminishing abrasion due to the contact or sliding of the head thereon or preventing contamination of the disk surface.
The carbon protective film is produced generally by the sputtering process or CVD process. Since the disk surface is protected with the two films, i.e., the carbon protective film and the lubricant film thereover, the interaction between the carbon protective film and the lubricant is important.
The lubricants generally in use are perfluoropolyethers having functional groups. Examples of functional groups are hydroxyl, amino and cyclophosphazene group. More specific examples of lubricants having hydroxyl include Fomblin Ztetraol manufactured by Solvay Solexis Inc., PHOSFAROL A20H, PHOSFAROL ADOH, PHOSFAROL A20H-DD, each product of MRESCO Corporation. As mentioned later, these four lubricants are low in resistance to Lewis acids. These are decomposed by Lewis acids and are concerned to accompany a great weight reduction by heat. They are to be improved in decomposition resistance.
PHOSFAROL A20H is a fluoropolyether having both hydroxyl and cyclophosphazene groups (Patent Literature 1). PHOSFAROL ADOH is a cyclophosphazene to which is bonded one perfluoroether chain having two hydroxyl groups (Patent Literature 2). PHOSFAROL A20H-DD is a cyclophosphazene to which are bonded two perfluoroether chains having two hydroxyl groups (Patent Literature 3).
Fomblin Ztetraol exhibits good properties to adhere to the disk due to the presence of the hydroxyl at opposite terminals of the molecule, does not spatter even when the disk rotates at a high speed and is capable of retaining the form of a lubricant film. However, this perfluoropolyether compound is low in resistance to Lewis acids, reacts with Al2O3 in the component of the head and becomes cleaved in its main chain (see, for example, Nonpatent Literature 1). When this cleavage progresses, the compound becomes lower in molecular weight, eventually vaporizing off from the magnetic disk surface and failing to retain the form of a lubricant film in systems involving contact or sliding movement between the head and disk.
On the other hand, PHOSFAROL A20H, PHOSFAROL ADOH or PHOSFAROL A20H-DD has at one terminal of the molecule a hydroxyl group contributing to the adhesion to the disk and at the other terminal a cyclophosphazene group for inhibiting the cleavage of the perfluoropolyether main chain with Lewis acids. Accordingly, the lubricant does not spatter even if the disk is in high-speed rotation, the compound is inhibited from decomposing with the Al2O3 in the component of the head and the lubricant is capable of retaining the film form. However, these PHOSFAROLs, similar to Fomblin Ztetraol, have a main chain which is low in resistance to Lewis acids, and are insufficient to restrict cleavage of its main chain due to the presence of a cyclophosphazene group, and react with Al2O3 in the component of the head and becomes cleaved in its main chain. When this cleavage progresses, the compound becomes lower in molecular weight, eventually vaporizing off from the magnetic disk surface and failing to retain the form of a lubricant film in systems involving contact or sliding movement between the head and disk.    [Patent Literature 1] Jap. Patent No. 4137447    [Patent Literature 2] Jap. Patent No. 4570622    [Patent Literature 1] WO 2008/140121    [Nonpatent Literature 1] Macromolecules, 1992, Vol. 25, pp. 6791-6799
An object of the present invention is to provide a stable compound which remains free of decomposition even when brought into contact with the magnetic head, a lubricant using the compound and magnetic disk having the lubricant applied thereto.